SDHI fungicides were discovered more than 40 years ago. Due to the limited disease and application spectrum of the “first generation” carboxamides, resistance under commercial conditions remained limited to a few crop/pathosystems (primarily Basidiomycetes), e.g.Puccinia horiana, chrysanthemum rust, and Ustilago nuda, loose smut in barley.

In addition to these “first generation” molecules, SDHIs with increased spectrum and potency were launched starting in 2003 and new ones continue to be launched today. These modern generation SDHIs are rapidly achieving market share in many crops and new SDHIs are currently in development.

The target enzyme of SDH inhibitors is succinate dehydrogenase (SDH, so-called complex II in the mitochondrial respiration chain), which is a functional part of the tricarboxylic cycle and linked to the mitochondrial electron transport chain (Keon et al., 1991). SDH consists of four subunits (A, B, C and D) and the binding site of the SDHIs (the ubiquinone binding site) is formed by the subunits B, C and D. Target site mutations conferring reduced sensitivity can develop in all three subunits.

Mutations conferring resistance

Mutations conferring reduced sensitivity were (and continue to be) identified in a number of pathogens both from field monitoring as well as laboratory mutagenesis studies. Field populations most intensively studied to date include Botrytis cinerea isolated from a number of crops (Yin et al. 2011, Veloukas et al. 2011), cucurbit diseases (McGrath 2008, Miazzi & McGrath 2008, Stevenson et al. 2008, Miyamoto et al. 2009, Ishi et al. 2011, Avenot et al. 2012), or Alternaria species on different crops (Avenot et al. 2008 and Avenot et al. 2009, Gudmestad et al. 2013). The market introduction of new SDHIs and more scientific studies will most likely change our current understanding of resistance in these field populations.

Work with isolates from both field and lab studies suggests that cross-resistance patterns between SDHIs for different target site mutations are complex. Different target site mutations confer varying degrees of insensitivity between the different SDHIs. This suggests the effect of these target site mutations on field performance of specific SDHIs may vary if they were to spread in field populations. The various degrees of reduced sensitivity to different target site mutations may be explained by structural differences between classes of SDHIs and how they interact with the target site of a specific pathogen (Scalliet et al. 2012). Additionally, the magnitude of the effect conferred by a specific target site mutation can vary from species to species. In other words, the reduction in sensitivity conferred by specific target site mutations may vary between pathogen species, SDHI used and geographic location of the isolates (Sierotzki and Scalliet 2013).

Current Resistance Status (2014)

In pathogens where isolates with reduced sensitivity were detected under field conditions, the current resistance status of the population is shifting as disease pattern/severity and product usage across multiple modes of action changes from year to year. Monitoring at the regional level in specific crop/pathosystems is crucial to understand the resistance status at any given time. As mentioned above, it is also important to understand how the different members of the SDHI group will respond to specific target site mutations within that species and the frequency of those mutations in the field population when deciding on specific product usage.

Isolates with reduced sensitivity were identified in field populations of Alternaria alternata on nut crops in the US (Avenot et al., 2008 and 2009), A. solani on potatoes in the US (Gudemstad et al., 2013), Botrytis cinerea from apple (Yin et al., 2011), kiwi (Bardas et al., 2010), and strawberry (Veloukas et al. 2011 and 2013), Corynespora cassicola on cucurbits in Japan (Miyamoto et al., 2009 and Ishii et al., 2011), Didymella bryoniae on cucurbits in the US (Avenot et al., 2012), and Podosphaeria xanthii in the US (Miazzi and McGrath 2008) and Japan (Miyamoto et al, 2010 and 2011). Reduced field efficacy of certain SDHIs was reported for all of these species.

In addition, field isolates with target site mutations conferring reduced sensitivity were identified during routine monitoring in Europe in B. cinerea in grapes, Venturia ineaqualis in apples, Pyrenophora teres in barley, Mycospharella graminicola in wheat and Sclerotinia sclerotiorum on oilseed rape. Many of the identified mutations have low to moderate resistance factors for commercially available SDHIs and the frequency of the resistant mutations remains low in the population. Reports on field performance of the SDHIs remained good in 2014 (MInutes of the 2014 SDHI Meeting, Recommendations for 2015). These early monitoring reports of isolates with reduced sensitivity emphasize the need to abide by resistance management guidelines to prolong the life of the SDHIs.